QAM modulation has been widely employed in many wired/wireless standards, such as LTE® and WiFi®. It is well known that inter-cell interference (ICI) in conventional cellular networks employing orthogonal frequency division multiple-access (OFDMA) with QAM tends to approach a Gaussian distribution. Recently, FQAM has been proposed in a downlink cellular OFDMA network instead of conventional QAM and it has demonstrated a significant performance gain for an interference-intensive scenario. The reason for the gain is because the distribution of the ICI plus additive noise (hereafter denoted as ICI for simplicity) received at the victim cell deviates from Gaussian distribution when FQAM is employed at the interfering cells, known as aggressors. Since Gaussian distribution of the noise is the worst-case distribution in wireless networks with respect to the channel capacity, the performance of the victim cell can be improved due to the deviation from the Gaussian distribution. In other words, performance for a user experiencing heavy interference from neighboring aggressors can be boosted if the aggressors utilize FQAM instead of QAM.
However, FQAM cannot simply be used constantly, since it introduces other drawbacks, such as decreased data throughput, so it should be deployed selectively.
The distribution of ICI in FQAM depends largely on the number of aggressors. The lower the number of aggressors, the larger is the deviation from Gaussian distribution, so that the greater the performance improvement at the victim cell is. With an increased number of aggressors, the distribution of FQAM ICI asymptotically approaches Gaussian distribution, according to the central limit theorem. Thus the capacity improvement is no longer significant.
In order to make the ICI deviate from Gaussian distribution as far as possible, all aggressors should employ FQAM. However, as mentioned previously, FQAM only achieves improved performance when the mobile device or User Equipment (UE) experiences high level of interference. For those UEs experiencing medium or low level of interference, QAM modulation outperforms FQAM. In this regard, when there are multiple UEs with different interference level but co-existing in the same cellular network and sharing the same resources, the resources allocated to FQAM and QAM should be orthogonal. In the context of this application, ‘orthogonal’ refers to the ability of two or more modulation schemes to co-exist in some way, without interfering with one another. For instance, as an analogy, literal orthogonality is provided in an optical system whereby horizontally and vertically polarized light signals may co-exist in space and time, without interfering. This is a literal orthogonal relationship. In the radio frequency domain, the term orthogonal refers to some property which provides the same degree of isolation without requiring any form of literal orthogonality.